1. Field of the Invention
The present invention concerns crosslaminates, i.e. laminates of films of which at least two are uniaxially or unbalanced biaxially oriented, in which the main direction of orientation in one of these films crosses the main direction of orientation in the other one.
More specifically the invention concerns modifications, made in a suitable pattern, of the surface properties of the two films on the surfaces which are inside the laminate and are bonded to each other. There are two different practical purposes of making such pattern formed modifications.
2. Description of the Related Art
One well known modification of internal surfaces in a laminate (although not actually used for crosslaminates) consists in printing a text or decorative pattern on one of the surfaces which become internal. Thus the text or pattern cannot be rubbed off the laminate during use.
One aspect of the invention is an improvement of this method, but limited to decorative, sales promoting patterns of crisscrossing colored bands by which the relatively expensive printing process is substituted by a low cost modification of the extrusion process. Furthermore a special embodiment of this aspect of the invention gives very special three-dimensional effect as it shall be described later.
As regards the importance of visual effects in products made of plastics, reference is made to an article in Modern Plastics December 2002, pg. 50: “Visual Effects means Business”, which states “(i)nstead of considering an exterior simply as a cover for components, manufacturers are using it as a marketing tool to differentiate products and allow personalisation”.
Modifications of the internal surfaces in a crosslaminate, made in a suitable pattern, can also, as it has been proposed, be used to improve the tear propagation resistance. This shall now be dealt with in detail, and in that connection a general explanation of the known crosslamination technology will be useful. Also for this aspect of the invention, the main purpose of the invention is to substitute relative expensive or less efficient process steps by a low cost modification of the extrusion method.
Cross-laminates of oriented films from synthetic polymer materials have been commercially produced since 1968, then mainly as described in the inventor's patent GBA-0 792,976 of May 23, 1955. To the inventor's knowledge the total annual worldwide production today amounts to about 30,000 tons. The cross-laminate is used in particular as industrial bags, coversheet, tarpaulins, pond-liners and similar products.
The polymer materials used for these cross-laminates have been mainly and are mainly polyethylene and polypropylene of different types, often modified by blending, and the old and present manufacturing processes comprise the steps of extruding a tube, which, by the draw-down, is oriented mainly in its longitudinal direction, helically cutting this tube to a web with its main direction of orientation on the bias, and continuously laminating two or more such webs with their main directions of orientation crisscrossing. There can also be included in the laminate a film which is oriented mainly in its longitudinal direction.
In the first commercialised technology based on these principles, the extruded tubular film, which is melt-oriented mainly in its longitudinal direction, is further cold stretched in this direction prior to the helical cutting. In a later commercialised technology, disclosed e.g. in U.S. Pat. No. 4,039,364 (Rasmussen), each tubular film is coextruded, having a layer which contributes mainly to the tensile strength in the laminate (hereinafter “the main layer”) and at least one surface layer (hereinafter “the first bonding layer”) adapted to help in the bonding of the films, which at least partly takes place by pressure and heat.
Also special layers are coextruded on the films, which become exterior in the laminate. These special layers are adapted to modify the surface properties of the laminate, especially for improved heat-sealing. In this later technology the helical cutting takes place in direct succession to the coextrusion without any cold stretching between, but in a separate production line. However, further stretching is carried out when the films have been brought together in a sandwich arrangement, bonded or not yet bonded to form a laminate. The films are biaxially stretched at a relatively low temperature.
The transverse component of this biaxial stretching takes place between grooved rollers.
In U.S. Pat. No. 5,028,289 (Rasmussen) and U.S. Pat. No. 5,626,944 (Rasmussen) this stretching between grooved rollers has been further developed.
Practical ways of carrying out the helical cutting are disclosed in U.S. Pat. No. 5,248,366 (Rasmussen). This patent also mentions an alternative cutting technique, namely that the tubular film can be provided with a helically extending melt orientation while it is drawn off from the coextrusion die, established by a relative rotation between the exit and the die, and subsequently the cutting may be parallel with the axis or may be at an angle to the main direction of orientation. The process may even be adjusted to produce a web in which the main direction of the melt orientation will become perpendicular to the longitudinal direction of the web.
For the sake of completeness it should also be mentioned that, in very early patents, there is also disclosed the possibility that longitudinally orientated polymer film material can be discontinuously cross-laminated and bonded in a press.
In a process which is entirely different from that described above, cross-laminates of a very stiff character are made for use in special, advanced products. They consist of polymers which in molten or part-molten state are fluid crystals, and which become oriented and cross-laminated already within the extrusion die by means of counter-rotating dieparts. However, this type of process and product is not a subject of the present invention.
Reverting to the other type of cross-laminates, which more commodities or technical products, they are especially characterized by high puncture strength and high tear propagation resistance. The heat-seal strength in a shear-type seal is adequate when a suitable lower melting polymer has been chosen for the surface layers of the laminate, while very special precautions must be taken if good shock-heat-seal strength is requested in peel-type heat-seals, as usually needed for industrial bags supplied with such heat-seals. These precautions are disclosed in the inventor's patent publications U.S. Pat. No. 5,205,650 and WO-A-98/23434.
As mentioned above the cross-laminates can exhibit a particular high tear propagation resistance, however this is under the condition of a generally low bonding strength. Due to the unbalanced orientation in the individual films and the criss-crossing of the main directions of the orientation, one film will have a tendency to propagate the tear in one direction and another film will tend to propagate the tear in another direction. Thereby there will be a tendency to eliminate the bonding at the location where the forces are concentrated, and if this tendency is sufficiently pronounced, the tear will “fork out” under a local delamination, and the “notch effect” of the tearing will almost be eliminated.
Hereby there will, generally speaking, be “competition” between the adhesive forces which try to withstand delamination, and the cohesive forces in each film which try to avoid a rupture or flow along any direction which is not parallel with the main direction of orientation. The said adhesive forces are (still generally speaking) independent of the thickness of the films, while the said cohesive forces are mainly proportional to the film thickness, when all other parameters are unchanged. As a consequence of this “competition”, “thin” crosslaminates will either exhibit a relative poor tear propagation resistance or a relatively high tendency to delamination. This is much less of a problem for crosslaminates of “thick” layers. For bags this “competition” will usually not cause any problems since filled bags are usually not subjected to delaminating forces, which means that a low bonding strength can be chosen, but the matter is very important for tarpaulins, cover sheets and similar products which will be subjected to repeated flexing during use, e.g. will flap in the wind. As a matter of practical experience the inventor and his licensees have found that in a tarpaulin made from a two-film crosslaminate based on combinations of LLDPE- and HMWHDPE-types, each of the films must be of a gauge of at least 45-50 gm−2, otherwise either the bonding strength or the tear propagation resistance will be unacceptable to the users. These experiences concern tarpaulins for “static” uses where there will not occur much flapping in the wind. For “dynamic” uses such as cover over trucks or goods waggons, where the tarpaulin will be subjected to a strong, repeated flapping, the gauge required is much higher.
One objective of the present invention is to solve this problem, so that high tear propagation resistance and an adequate bonding between the films can be achieved at the same time and in a practical way, even in crosslaminates of low gauge.
In connection with the solution of the above mentioned problem, the inventor has constructed a circular coextrusion die capable of coextruding, in a practical way, an array of strands on a tubular film, and this construction is also an objective of the present invention.
In GB-A-1,095,479 of Mar. 3, 1964 (assigned to Metal Containers) the inventor suggested that the problem which has been identified above, can be solved by strongly welding the films to each other in spots or lines and weakly welding them together over the rest of the contacting surfaces (a strong bond/weak bond generally being better than a strong bond/no bond). This enables the tear to “fork-out” as described above in the weak-bond areas, while an overall delamination is prevented by the strong-bond spots or lines.
For the strong welding, the patent suggests heating, ultrasonic welding, application of a solvent (preferably hot vapours) to dissolve a thin surface layer, or using quickly polymerizing monomers acting as strong binders. For the weak welding the patent suggests (using polyethylene crosslaminates as an example) to apply a gel of low molecular weight polyethylene or paraffin wax, which has been dissolved e.g. in toluene or xylene by heating and has formed gel by cooling. A thin layer of this gel including the solvent is selectively applied by printing technique before the strong welding is carried out by blowing vapours of toluene or xylene towards the film surfaces while they become united between rollers. Alternatively there is added a minor amount of a slip agent to toluene or xylene, and this “contaminated” solvent is used in similar manner to the gel.
DK-A-1017/67 (de Pont) published on Feb. 24, 1967 claims crosslaminates of films bonded in spots or lines, (which may be two arrays of lines forming a net pattern) while the rest of the contact area is (quoting the main claim) “practically not bonded”. Three methods of carrying out the bonding in spots or lines are disclosed. One consists in applying a caoutchouc-like binder in the desired pattern. This application is said to take place by well known methods, but it is not further explained.
A second method consists in treating the selected areas of a surface on one of the films which chlorine, followed by lamination under pressure at an elevated temperature below the melting point of the film material.
A third method, which is described as being preferred, is carried out by treating the selected areas of a film surface with a corona discharge, followed by lamination under pressure at an elevated temperature under the melting points of the film material. In this case a roller formed electrode, connected to earth, is supplied with the desired pattern (which may be a net-pattern) so that the electrical discharge only takes place in the space determined by this pattern. The matching film surface is corona treated over its entire area. It is indicated that this treatment requires an effect of 20 W cm−1 width if the velocity is 0.5 m min−1.
In the above-mentioned latter patent U.S. Pat. No. 4,039,364 (Rasmussen) in which there is coextruded a surface layer on each oriented film (“the first bonding layer”) to enhance and control the bonding, a strong bond/weak bond adhesion system is established by using different lamination temperatures at the different locations of the laminate. Thus in example 1, by the use of coextrusion and helical cutting three films are made with different direction of melt orientation and surface layers of EVA to assist the lamination (in the foregoing called “the first lamination layer”). There is established a weak bonding simultaneous with transverse orientation, by taking a sandwich of the three differently oriented films seven times through a set of intermeshing grooved rollers. The pitch of these rollers is 1.5 mm, of which the width of the groove amounts to 1.0 mm and the width of the circular “tooth” to 0.5 mm. Between each passage through grooved rollers, the pleats formed in the film sandwich are straightened out.
These stretching steps take place at 20° C. but still produce some bonding (peel strength 10 g cm−1) due to the intimate contact between the films and the effect of stretching them together. After the seven passages at 20° C. the film is passed once through a similar set of grooved rollers with the same dimensions and intermeshing, but heated to 120° C., whereby there is formed lines of strong bonding. Finally the laminate is longitudinally oriented.
In EP-A-0 099 222 (Mercer et al) of Apr. 7, 1983, orientation and crosslamination in a spot-welded pattern is carried out as a unitary process in and immediately following a circular die with two counter rotating dieparts. Each of these dieparts produces a film supplied with an array of ribs, arranged so that the two arrays face each other. Due to the counter-rotation, the melt orientation in and the array of ribs on one of the ribbed films become right-handed and for the other become left-handed. The two arrays of ribs are brought to meet each other at or immediately after the die exit, and bonding takes place only in the spots where the ribs intersect each other. The ribs keep the two spot-welded films spaced apart from each other also in the final product.
Melt-orientation with crisscrossing orientation takes place while the polymer material flows through the two counter-rotating parts and by the blowing and longitudinal drawdown when the laminate has left the exit of the die. There is no subsequent orientation process carried out.
The process is not a coextrusion process. The films and the ribs consist of the same polymer material and come from the same extruder.
To the knowledge of the inventor, none of the above mentioned methods of making strong bond/weak bond or strong bond/no bond adhesion patters in crosslaminates has ever been used for commercial production although the principal, great advantage of such bonding systems in crosslaminates has been recognised for about 40 years. However, each of the proposed methods have serious drawbacks. The methods which make use of organic solvents for polyolefins, especially in vapour form, are connected with health hazards unless very expensive machinery is used, not least because it is difficult to avoid traces of the solvent to remain in the final product.
The proposed corona treatment in a pattern, followed by lamination under pressure and heat but below the melting point of the polymer material, is applicable only if the production capacity is very low. In commercial production of crosslaminates for commodity uses, such as e.g. tarpaulins and cover sheets, the lamination velocity must be about 60 m min−1 or more and the width about 150 cm or more. Using the above mentioned information about power consumption, the 60 m min−1 and 150 cm will require 900 kW, which of course is not practically possible. Nor is treatment with chlorine in a pattern a process suited for industrial production on a large scale.
The use of binders, applied by printing technique from a dispersion or solution, requires a previous strong surface treatment, when the polymer material is polyethylene or polypropylene, normally a very strong treatment by corona, and therefore this method is not economical either.
A strong bond/weak bond or strong bond/no bond pattern achieved by different temperatures will inevitably create differential shrinkage if the pattern is a line or pattern (including a net pattern), and this makes the crosslaminate look untidy. Differential shrinkage can be avoided if the areas of strong bonding are small dots, but in this case the product gets a dotted appearance which may be unpleasant.
Furthermore the apparatus needed for adequate heating in a spot pattern to a controlled temperature is relatively complicated, when the velocity is high, since the laminate must maintain contact with hot spots on a heater over a long passage without any displacement of the laminate taking place in spite of its tendency to shrinkage.
In the unitary crosslamination process with counter-rotating dieparts it is, from the point of view of strength, a drawback that film forming and molecular orientation are so closely coupled together. This makes it virtually impossible to tailor-make the properties for different purposes. Furthermore the inventor has found that a crosslaminate which is entirely unbonded except in spots, exhibits a relatively low yield point and high tendency to creep in a direction between the main directions of orientation in the two laminated films.